In the state-of-the-art, treatment of groups of semiconductor substrates is well-known. Such a process should be distinguished from systems wherein the wafers are handled separately, that is, one after the other and not simultaneously.
It is clear that a system wherein a large number of wafers (up to several hundreds) are simultaneously treated in a boat, results in a considerable time advantage per wafer, especially for long processes. Naturally, the aim is to treat each of the semiconductor substrates identically, as far as is possible, so that after the treatment all products have been treated in the same way and to the same extent.
One of the problems which can occur is the exhaustion of the treatment gas, which is introduced at one side of the oven. When a layer is deposited by the treatment gas, for example, this layer will generally be thicker at the inflow side than at the side where the gas is removed. This effect can be compensated for by choosing a temperature several degrees higher on the outflow side than on the inflow side. It will be clear that the application of considerable temperature differences in the oven is undesirable, since a difference in treatment does then occur. A particular case occurs with the depositing of silicon dioxide layers from Tetra Ethyl Ortho Silicate (TEOS) vapor. In this case it has been found that, for a relatively high process pressure, high temperature and low TEOS gas flow, the layer thickness on the gas outflow side is smaller than on the gas inflow side, as is to be expected due to the exhaustion of the treatment gas. Nonetheless, for low process pressure, low temperature and high TEOS gas flow, the layer thickness on the outflow side can be thicker than on the inflow side. Likewise, it has been observed that the layer thickness at the edge of the wafer, compared with the layer thickness at the center of the wafer, increases in the direction of the outflow side: depending on the chosen temperature, pressure and TEOS gas flow, the substrate near to the gas inflow side can show a pronounced convex profile in the layer thickness, that is, thinner at the edge than in the center, while under the same circumstances, a substrate near the gas outflow side can show a pronounced concave profile.
The result is that no uniform treatment takes place and, for certain applications, parts of the boat are not filled with wafers, because such an uneven deposition and reaction respectively take place either in the top part or in the bottom part of the boat, compared with the other parts of the boat, that semiconductor substrates are produced which are no longer usable.
The decomposition of TEOS is described in J. Electrochem. Soc., Vol. 140, No. Oct. 10, 1993, page 2952, by T. Sorita, S. Shiga, K. Ikuta, Y. Egashira and H. Komiyama. These authors suggest that the decomposition of the TEOS molecule takes place via the forming of intermediate radicals in the gas phase. These intermediate radicals in turn attach themselves to the wafer and form the silicon dioxide layer. The decomposition rate of the silicon dioxide layer is directly related to the concentration of the intermediate radicals in the gas phase. To achieve a uniform layer thickness over the whole series of semiconductor substrates, the concentration of these intermediate radicals must be constant across the whole reactor. This concentration is determined by the balance between the rate with which the intermediate radicals are formed and the rate with which they are consumed by deposition on the substrates. The rate of the forming is determined by the process conditions, such as pressure, TEOS concentration, temperature and the concentration of already present intermediate radicals. The above-mentioned process results can now be explained as follows. At low pressure, low temperature and high TEOS gas flow, the residence time at the inflow side was too short to build up a sufficient concentration of intermediate radicals. Consequently, under these circumstances, the layer thickness of the inflow side is relatively thin and the layer thickness at the edge of the substrates is relatively thin. During the inward diffusion, a higher concentration of intermediate radicals already forms whereby the layer thickness near the center of the substrate increases. Near the gas outflow side, a high concentration of intermediate radicals has already formed while the exhaustion of the TEOS itself is relatively small. This results in higher deposition speed and a relatively thicker edge. At high pressure, high temperature and low TEOS gas flow, the forming of the intermediate radicals is no longer a limiting factor as the residence time of the gas in the reactor is increased. In this case, the detected layer thickness will show the usual exhaustion characteristics of TEOS: thinner towards the center of the substrates and thinner towards the gas outflow side.
It is noted that the rate of forming of the intermediate radicals in the gas phase is a volume measure. With a higher gas volume, an proportionally larger number of intermediate radicals will form in absolute terms. The `consumption` of intermediate radicals is determined by the conversion rate in the silicon dioxide layer, and this depends on the temperature and the concentration of the intermediate radicals present. For the decrease in absolute terms, it is the amount of available surface on which the silicon dioxide can form that is important. To summarize, it can be concluded that for a uniform process result, it is important that a balance exists throughout the reactor between the forming of the intermediate radicals, which increase proportionally with the locally available gas volume, and the consumption of the intermediate radicals, which increases proportionally with the available surface. This surface is made up of the surface of the process tube, the boat and the substrates. A balance between volume and surface is thus of fundamental importance. Near the center of the substrate, the ratio of surface to volume is entirely determined by the distance between the substrates. At the edge of the substrates, besides the distance between the substrates, the amount of space between the substrate edge and the process tube is also of importance.
In the abstract of the Japanese patent application 56155529, a boat with a variable pitch is described. With this, according to the description a more even thickness is achieved. In this system the substrates are loaded by hand.
In the U.S. Pat. specification No. 5,217,560, gas or radicals are introduced over the height of the boat from the outside by various openings, to achieve an equal concentration in this way. In this manner, an optimization of the uniformity of the layer thickness can be achieved from the inflow side to the outflow side, but not from the edge to the center of the wafer. Furthermore the complexity of the system increase by the extra gas supply openings and, when used, the amount of gas to be supplied to each of the individual gas openings must be determined and controlled. Such a process with so many parameters is more liable to faults and is undesirable in production circumstances.